It is common practice to fabricate large cylindrical vessels or tanks by rolling flat sheets or plates into cylinders, welding the longitudinal seam where the edges of the plates abut, and then joining two or more such cylinders end-to-end by welding the circumferential seam of adjacent cylinders where their respective edges abut. Typically, during the welding operation the cylinders are rested atop a number of axially spaced pairs of wheels having axes parallel to the axis of the cylinder, so that by turning the wheels, by, e.g., an electric motor and suitable control circuitry, the cylinder turns, and the seam between the cylinder sections may be welded with a stationary welding machine, or by a worker operating manual or semi-automatic welding equipment. It will be apparent that moving the work past the weldor at the proper speed will allow him to work far more efficiently than if he is forced to clamber over it. Further, by so doing, the welding operation can always be conducted at a single optimum orientation with respect to the horizontal, so that the parameters of the welding process are uniform throughout the operation. Such powered turning rolls are well-known in the art (See, for example, U.S. Pat. No. 3,448,877 and "Bulletin TR 77," a catalog of such equipment published by the Aronson Machine Co., the employer of the present inventor); a modification to the standard system, in which the workpiece rests upon a resilient belt rather than upon the wheels, for protection of its surface, is shown in U.S. Pat. No. 2,669,364, and on page 14 of the aforementioned catalog. A problem which frequently arises is that the cylindrical sections to be joined together are not exactly the same diameter, or are not perfectly round. In order to fabricate the strongest possible joint, it is clearly advantageous to distribute the total error around the whole circumference of the cylinders, so as to make the average error as small as possible. Thus, if two weldments of nominally 4 meters diameter to be joined differ in diameter by 20 mm., and they are butted together flush at one point, as for example, by resting the ends of the cylinders to be welded upon parallel cylinders or beams bridging the joint, their edges will be 20 mm apart at the opposite point; but by moving one cylinder 10 mm with respect to the other, the error can be limited to 10 mm. As the workpieces are often very heavy, up to one thousand metric tons, it is apparent that there exists a need for powered apparatus whereby such adjustments can be made safely, efficiently and in small increments.
Several types of fit-up rolls have been made to this end in the past. Typically these devices are used in conjunction with powered turning rolls, but are not themselves powered to rotate the cylinders past a stationary welding machine. Rather, one of the cylinders to be joined rests upon powered rolls, the other upon idler rolls adapted to be adjusted so as to position the cylinder. The present invention is of this type as well.
When the two cylinders are brought into alignment, they are "tacked" (i.e. welded together manually at a few points along the seam). Turning one cylinder by means of the powered rolls then, of course, turns the other, and stationary welding equipment, either automatic or manually operated, may be employed to finish-weld the seam.
Two types of fit-up rolls have been made or disclosed in the past. One moves the idler wheels horizontally, toward or away from each other, which has the effect of raising or lowering a supported cylindrical workpiece, or moving it horizontally; alternatively, one or both of the wheels can be moved vertically, thus also raising, lowering, or transferring the cylindrical workpiece; the wheels are moved by powered leadscrews. U.S. Pat. No. 3,514,090 shows a leadscrew system in a different application which is analogous to that used in fit-up idler roll systems.
There are deficiencies inherent in both sorts of leadscrew-powered idler roll systems, including the fact that they are limited to approximately 60 metric tons capacity, due to the excessive power required to raise heavier workpieces by means of leadscrews. As leadscrews are essentially friction devices (i.e., the entire weight of the workpiece rests upon the screws; thus the friction between the screw and its associated nut is directly proportional to the weight of the workpiece) the power requirements are excessive. Moreover, the leadscrews are expensive to manufacture and are not durable in service, again due to the excessive power requirements.
A second type of fit-up idler roll system is disclosed in U.S. Pat. No. 3,480,158. These idler rolls are mounted on pivoted arms, so that their position can be adjusted by actuation of an hydraulic cylinder acting on the arm. This type of system is also limited, by the excessive power required to energize the hydraulic cylinders, to approximately 60 metric tons capacity.